Global Forecast for the 21st Century

Projected changes in surface temperatures comparing data collected between 1961-1990 and 2020-2029, or between 1961-1990 and 2090-2099, under emissions scenarios B1, A1B, and A2. From IPCC 2007.

Average global temperatures are anticipated to warm by somewhere between 1.5°C and 6.8°C from 2000 to 2100, depending on human activities . These changes will not be uniform, although no regions are expected to cool. Landmasses will warm more than the oceans, and Arctic regions will warm more than other parts of the world, perhaps by as much as 8°C or 9°C above 1975 temperatures. This will melt more of the northern ice and snow pack, thereby decrease albedo of these regions, and accelerate warming.

Sea ice in the polar oceans is melting and may disappear entirely during summer months. This will shorten sea routes, say, from Europe and the east coast of the United States to China. It will also make more accessible some natural resources such as oil deposits in the Arctic seabed. Sea level is predicted to rise as higher temperatures expand the volume of ocean waters and melt the snow and ice situated on landmasses, particularly Greenland and Antarctica. Over 20 different GCMs predict that the sea level will rise by 0.2 m to 0.5 m from 2000 to 2100. [1] These models usually assume that glaciers will keep flowing (moving) at their current speed. Recent observations in Greenland have shown that glacial flows are accelerating. [2], [3], [4], [5] The likely mechanism is that as temperatures increase, the ice along the edges of a glacier melts. This water makes it way to the bottom of the glacier and acts as a lubricant that accelerates the ice flow. An alternative forecast, which takes this phenomenon into account, predicts an even greater change in sea level: the sea level in 2100 will be 0.5 m to 1.2 m higher than in 2000. A rising sea level will put coastal regions of the world at great risk. Cities with large populations, such as those along the Gulf and East coasts of the United States, will be inundated. Bangladesh, one of the world’s poorest yet most populous countries, is perhaps the most vulnerable to sea-level changes: about 80% of this country is below 10 m in elevation, and so a 0.5 m to 1.0 m rise in sea level will permanently flood between 6% and 10% of its land area and displace between 3.4 million and 17 million people. [6], [7] In addition to worldwide flooding, the rising sea level may contaminate the freshwater supplies of many coastal regions with seawater.

Global warming will promote greater evaporation of water from Earth’s surface; therefore, global precipitation should increase on average. In particular, polar and equatorial regions are likely to become wetter. The mid-latitudes, however, will become drier, especially areas that are already deserts.

Hotter and drier weather at the mid-latitudes will foster more frequent and more severe forest fires.[8] Forests in the western United States contain from 20% to 40% of the carbon sequestered in the vegetation of the entire country. Fires might turn these forests into sources of additional CO2 that further accelerate global warming, rather than sinks that mitigate it. Clearly, more intensive management of these lands will become necessary.

Higher temperatures and lower precipitation at mid-latitudes will also decrease the snowpack in many mountain ranges. For example, the Sierra Nevada Mountains in California may receive from 30% to 90% less snow by the year 2100. [9] Also, only about one-third of the ski resorts in Europe will have reliable snow by 2100. [10] Finally, even in areas where precipitation does not change significantly, the amount of water available to living organisms, lakes, and rivers will decrease because at higher temperatures, more water will be lost to evaporation.

As CO2 concentrations in the atmosphere increase by 50% to 300% during this century, more CO2 will dissolve in the oceans and form carbonic acid. This carbonic acid dissociates in water to release a proton and bicarbonate. Higher atmospheric CO2 concentrations and warmer temperatures increase proton concentrations in the oceans (i.e., lower the pH), and oceans are thereby becoming more acidic. Simulations on GCMs affirm this trend. They indicate that the oceans in preindustrial times were more alkaline, with a pH of 8.25 (a pH of 7.0 is neutral); they dropped to pH 8.15 by around 1994; and they are expected to reach pH 7.85 by 2100. [11], [12] The pH scale is logarithmic, so a shift in ocean pH from 8.25 to 7.85 means a 250% increase in proton concentrations, a change that will have a strong effect on sea life.

The pH of ocean surfaces at different latitudes. Reconstruction of the pH in preindustrial times (mean +/- uncertainty, black +/- gray band) was delivered by subtracting anthropogenic contributions from current data. Data from 1994 and projections from three GCMs for 2100 under a scenario similar to A1T and another intermediate to A2 and B2. After Oer et al. 2005.

Warmer sea surface temperatures and cooler stratospheric temperatures accentuate the temperature gradients that empower major storms. Consequently, major storms are increasing in intensity. This, together with the rising sea level, exacerbates storm surges and threatens coasts adjacent to warm ocean currents. Projections about the frequencies of major storms, however, remain uncertain because of the complex nature of storm formation and will require additional data and fine-resolution modeling. [13], [14]